A mixer circuit used for a tuner integrated circuit (IC) for television (TV) and the like that frequency-converts an input signal is generally provided with an input signal port, an output signal port, and a local signal port. The mixer circuit outputs a signal having a frequency, which is difference or sum between/of a frequency (input frequency) of a signal (for example, a received TV broadcast signal) input from the input signal port and a frequency (local frequency) of a local signal input from the local signal port from the output port.
In general, the local signal includes not only a desired frequency (local frequency) but also a harmonic component. In the above-described mixer circuit, the input signal also couples to the harmonic. Therefore, the output signal also includes a signal in which the input signal couples to the harmonic to be frequency-converted.
When it is required to equally receive a frequency signal of a wideband as when receiving the TV broadcast, in the mixer circuit, it is required that the mixer does not respond to the harmonic component of the local signal.
A reason for this is hereinafter described by a specific example. For example, when a received frequency range is from 45 MHz to 1000 MHz, an electric wave of this wide frequency range is equally input to the mixer circuit. At that time, when a direct conversion system in which the input frequency and the local frequency are equal to each other and the output signal is a base band signal is used, the mixer circuit frequency-converts the input signal of which input frequency is 100 MHz using the local signal of 100 MHZ to output as the base band signal centered around 0 Hz. However, since the frequency range of the input signal is wide, an interfering wave of 300 MHz three times as high as the local frequency of 100 MHz is also input to the mixer circuit as the input signal, for example, to be coupled to a third harmonic (300 MHz) of the local frequency and is frequency-converted to a frequency band around 0 Hz as a desired base band signal. As a result, poor reception occurs.